The invention relates to biological processes for producing one or more desired products, including alcohols such as ethanol and butanol. The processes comprise carrying out first and second fermentations of substrates in first and second bioreactors, wherein each fermentation produces one or more desired products and/or one or more by-products that can be utilized in the other fermentation. A product and/or by-product of the first fermentation is introduced to the second bioreactor during the fermentation, and a product and/or by-product of the second fermentation is introduced to the first bioreactor during the fermentation.